So i recently bought a high top handicap van to convert into a tiny home bus sorts thing. The van already has an aux battery installed for the wheelchair lift, but i wanted to remove the lift and use the aux battery for everything else necessary, but I also wanted to mount solar panels.on the roof to run everything while not on the move. Is there anyway to just patch the panels into the battery and have it get charged by the car when driving and the solar when not? Any advice would be much appreciated.

The van has a control center connected to the aux battery for the lift and some lights and stuff, what would i need to do to get some regular outlets to run a mini fridge and charge my devices etc.

I am not looking to run too much. A 30 watt mini fridge, some couple devices (cell phone/ipad) and maybe a very low watt tv, along with some lights. If i can get away with just using one panel and the existing aux battery, I'd prefer that. I am just to use whatI got for now to run, essentially a mini fridge and some cell phones.

using the calculator in my previous link. that is a fridge @40w running 16 hours + 3 usb chargers running 5watts each for 12 hours + a tv 150 watts @ 12 hours. i came up with a daily total of watts of 2620.

divide that total by 12volts and that should be the amp hours of battery needed.

2620watts/12volts = 218.3 amp hours.

the depth of discharge for lead acid batteries is 50%, so multiply the amp hours by 2.

your battery bank to run that load would be 436 AH.

many people say to add a safety factor of 2, so make that 900AH +.

unless your 3rd battery is a 8D, you're probably not packing more than 100AH in a single group 31 battery. you probably have less than that.

i know nothing about solar and the time it would take to recharge that type of use.

Turf has the math right.
From the practical side, for my house batteries I Use two T105 deep cycle 6 volt golf cart batteries connected in series for a 12 v output. I can run my 36 watt mini frige via an inverter for about 7.5 hours before the inverter shuts down due to low voltage (that is, without discharging the batteries below the accepted amount). That said, I'd guess you'll need way more batteries than you have. Jack